(a) Field of the Invention
The present invention relates to an optical recording medium having a plurality of recording layers and, more particularly, to an optical recording medium having a plurality of recording layers consecutively arranged from a light-incident side of the optical recording medium.
(b) Description of the Related Art
Optical recording media include a phase-change optical recording disk (phase-change disk), wherein irradiation of a laser beam forms and erases a recorded mark on a recording layer in a reversible way to change the optical property of the recording layer between the recorded state and the erased state. Upon reproducing data recorded in the phase-change disk, a reproducing laser beam is irradiated onto the disk and the reflected laser beam is examined to detect the optical intensity or phase thereof for judging the data.
A multiple-recording-layer phase-change disk is known which includes a plurality of recording layers layered one on another for increasing the recording capacity. Data are recorded and reproduced to/from the multiple-recording-layer phase-change disk by irradiating a laser beam through one of the surfaces of the phase-change disk. A two-recording-layer phase-change disk, for example, theoretically has a recording capacity substantially double the recording capacity of a single-layer phase-change disk.
FIG. 4 shows a layered structure of a conventional two-recording-layer phase-change disk 50, which includes, consecutively from the light-incident side, a first disk section 51, an optical separation layer 52 and a second disk section 53. The first disk section 51 includes a substrate 55, and a combination of recording layer 56 and reflective layer 57, which are consecutively formed on the surface of the substrate 55 far from a light-incident side of the phase-change disk 50. The second disk section 53 includes a substrate 31, and a combination of reflective layer 59 and recording layer 60, which are consecutively formed on the surface of the substrate 58 near the light-incident side.
The recording layers 56, 60 are made of a material having a reversible phase-change property changing between a crystal phase and an amorphous phase, which have different optical transmission factors (referred to as simply transmission factors hereinafter). A laser beam 54 is irradiated onto the recording disk 50 so as to be focused in the vicinity of the recording layer 56 or 60, to obtain a reflected laser beam from the corresponding reflective layer 57 or 59. The reflected laser beam is examined for the optical intensity thereof to reproduce the data recorded on the recording layer 56 or 60.
In order for obtaining a sufficient optical intensity for the laser beam reflected from the second disk section 53 in the two-recording-layer phase-change disk 50, the first disk section 51 should have a higher transmission factor. This is because the optical beam reflected from the second disk section 53 passes twice through the first disk section 51. If the first disk section 51 has a transmission factor of T0 and the second disk section 53 has a reflectivity of R1, the reflected beam from the second disk section 53 has an optical intensity down to T02×R1 of the original optical intensity. Thus, it is desired that the first disk section 51 have a transmission factor of 50% or above for obtaining a practically sufficient optical intensity for the laser beam reflected from the second disk section 53.
In the conventional two-recording-layer phase-change disk 50, the reflective layer 57 in the first disk section 51 has a relatively small thickness to thereby suppress the reflection by the reflective layer 57. It may be considered to reduce the thickness of the recording layer 56 in the first disk section 51 in order to compensate a shortage of the transmission factor, instead of the smaller thickness of the reflective layer 57. However, the smaller thickness of the recording layer 56 in the phase-change disk 10 makes it difficult to crystallize the recording layer 56, thereby degrading the performance for forming and erasing a recorded mark on the recording layer 56.
Patent Publication JP-2004-310992A describes a technique for improving the transmission factor of the first disk section, wherein a single dielectric layer having a higher refractive index is provided on the surface of the reflective layer in the first disk section far from the light-incident side of the phase-change disk.
The technique described in the patent publication may improve the transmission factor of the first disk section by 2 to 10%, if the single dielectric layer has a refractive index of 2.0 or above. However, such an improvement is insufficient for the first disk section to have a transmission factor of 50% or above.